When producing a bodyshell of a railcar, typically, metal panels are joined to one another by welding. However, since the metal panel is easily distorted by the welding, a high degree of welding skill is required. In addition, since the welding is accompanied by electricity and heat, the metal panel to which fittings are attached in advance cannot be subjected to the welding. Here, proposed is a module construction method of coupling a plurality of side modules to one another by fastening members or the like to assemble a side bodyshell, each of the side modules being produced in advance to have a predetermined configuration (see PTLs 1 and 2, for example).